REVIEWS OF BOOKS.
The problem which the author undertakes to investigate is whether or not the logical inferences to be drawn from the various results of modern science lend support to the view that our earth is the only inhabited planet, not only in the solar system, but in the whole stellar universe. A thoroughgoing review of such a book should be undertaken only by an astronomer who possesses a first hand knowledge of the facts which form the ground of the many inferences of which Mr. Wallace's long and careful argument consists. It is possible, however, in viewing the subject from the standpoint of a layman, to judge as to whether or not the conclusions are justified by the premises, supposing of course that the premises rest upon undisputed facts. Therefore, without challenging Mr. Wallace's alleged facts and generalizations, it is a matter of considerable interest to inquire as to their bearing upon his ultimate conclusion that our earth is the only inhabitable planet within the vast stretches of the universe.
The argument is based upon the following considerations, which can be outlined here only in a very brief and general manner,—merely a rough sketch of the chief points of his position. The universe presents a unity of structure and arrangement. The stars are not infinite in number and extent, but fall within a single system. The earth occupies a central position within the stellar universe, whose outer bounds are marked by the enclosing circle of the Milky Way. Within the sweep of the solar cluster near the center of this vast system, all planetary motions are less rapid and more controlled, and therefore there is less danger of catastrophic collision, and greater stability of conditions is possible. Were the solar system nearer or within the bounding circle of the Milky Way, confusion and instability would prove wholly inimical to the evolution of organic forms of life, which require stable conditions continuing throughout unthinkable æons of time. Moreover, throughout the entire universe there is evidence of a mechanical, physical, and chemical uniformity. All living organisms such as appear upon the earth result from exceedingly complex combinations, adaptations, and adjustments within the scope of the well-known and recognized laws of nature. The conditions essential to life are solar light and heat, an adequate distribution of water upon
